Large quantities of flexible pipes are used within the oil industry, basically for offshore production, but frequently onshore as well. In particular for offshore production flexible pipes are in many cases the only solution. The connection between the fixed equipment placed on the seabed and the floating drilling or production units usually requires flexible pipes. There are basically two types of flexible pipes: “unbonded flexible pipe” as described in standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008 and “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and “bonded flexible pipe” as described in API 17K “Specification for Bonded Flexible Pipe” Mar. 1, 2002 and “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition.
The unbonded flexible pipe construction consists of separate unbonded polymeric and metallic layers, which allow relative movement between layers, in particular in the form of sliding between an armor layer and adjacent layers. The armor layers are usually mainly of elongate and helically wound elements where respective windings thereof can move relative to each other e.g. by sliding between adjacent windings.
In the bonded flexible pipes the steel reinforcement is integrated and bonded to a vulcanized elastomeric material such that the reinforcements become embedded into the polymer. Both constructions are spoolable; the bonded construction is generally used in short lengths up to 30-40 m, but it is normally available up to a few hundred meters in single pieces, which optionally can be assembled with intermediate couplings.
An unbonded flexible pipe usually comprises an inner liner, often also called an inner sealing sheath or an inner sheath, which is the innermost sealing sheath and which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or more armoring layers. Often the pipe further comprises an outer protection layer which provides mechanical protection of the armor layers. The outer protection layer may be a sealing layer sealing against ingress of sea water. In certain unbonded flexible pipes one or more intermediate sealing layers are arranged between armor layers.
In general flexible pipes are expected to have a lifetime of 20 years in operation.
Examples of unbonded flexible pipes are e.g. disclosed in U.S. Pat. No. 6,978,806; U.S. Pat. No. 7,124,780; U.S. Pat. No. 6,769,454 and U.S. Pat. No. 6,363,974.
In practice the known unbonded pipe normally comprises at least two armoring layers located outside an innermost sealing sheath and optionally an armor structure located inside the innermost sealing sheath normally referred to as a carcass, where the armoring layers and optional carcass are not interfacially bonded to each other.
The term “sealing sheath” is herein used to designate a liquid impermeable layer, normally comprising or consisting of polymer. The term “inner sealing sheath” designates the innermost sealing sheath. The term “intermediate sealing sheath” means a sealing sheath which is not the inner sealing sheath and which comprises at least one additional layer on its outer side. The term “outer sealing sheath” means the outermost sealing sheath.
The armoring layers usually comprise or consist of one or more helically wound elongated armoring elements, where the individual armor layers and elements thereof are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
A traditional prior art flexible unbonded pipe comprises from inside and outwards an optional carcass (sometimes also called an inner armor), an innermost sealing sheath, a pressure armor, a tensile armor and optionally an outer protection sheath for mechanical protection and/or for sealing against ingress of seawater in use. The pipe may comprise additional layers, such as anti-wear layers between armor layers, insulating layers, intermediate sealing layers and/or an anti-birdcage layer outside the outermost tensile layer to prevent the tensile armor layer from buckling.
The carcass has the purpose of protecting the innermost liner against compressive forces, either due to mechanical forces acting on the pipe or due to compressed fluids trying to squeeze the liner, and thereby preventing the pipe from collapsing e.g. when subjected to hydrostatic pressure. The carcass usually comprises helically wound and interlocked elongate armor elements. The innermost sealing sheath forms the innermost sealing sheath and defines the bore of the pipe.
The tensile armor is usually in the form of a plurality of tensile armor layers which are usually pair-wise cross wound (wound with opposite winding direction). The tensile armor layers are normally composed of elongate armor elements which are helically wound with a relative low winding angle relative to the axis of the pipe—e.g. about 55 degrees or very often less. The tensile armor has the purpose of providing the pipe with strength in its length direction and preventing undesired elongation of the pipe while still maintaining high flexibility.
The pressure armor is usually in the form of one or more layers composed of helically wound steel elements which are wound with a relatively high winding angle to the axis of the pipe. The pressure armor mainly has the purpose of protecting the pipe against internal pressure (from inside of the pipe and outwards) provided by the fluid transported in the pipe. Such pressure can be very high and often varies along the length of the pipe as well as over time. The pressure armor may to some degree also protect against external pressure (from outside of the pipe) in particular if the pipe has a liquid impermeable sheath on the outer side of the pressure armor.
Usually it is desired to interlock the windings of the armor elements of the pressure armor to avoid uncontrolled displacement of the windings along the length of the pipe and thereby control the maximal gaps between windings such as e.g. described in U.S. Pat. No. 5,813,439. Such undesired gaps between windings may result in lateral buckling and/or in damaging the innermost sealing sheath by entrapment in gaps of the windings. Lateral buckling may for example occur when the pipe is subjected to axial compression. This phenomenon is also called “reverse end cap effect” or “inverse bottom effect”. The interlocked windings are formed to have a ‘play’ which means that adjacent windings can move towards or away from each other at a certain distance to each other. The range of movement between such adjacent interlocked windings of armor elements is called a play. By interlocking adjacent windings of pressure armor elements it is ensured that the distance between such windings is limited to a selected maximum while allowing flexibility of the pipe.
However, it is rather expensive to interlock the windings of the armor elements of the pressure armor and it requires expensive production equipment.